1. Field of the Invention
The present invention relates generally to a cellular wireless communication system, and more particularly to a method and an apparatus for transmitting/receiving a DownLink (DL) Synchronization CHannel (SCH) for initial cell search and neighbor cell search.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) technology has being widely used in conjunction with broadcast and mobile communication technology. The OFDM technology eliminates interference between multi-path signals in wireless communication channels, guarantees the orthogonality between multiple access users, and enables efficient use of frequency resources. Therefore, OFDM technology is more useful in high speed data transmission and for a broadband system, as opposed to the Direct Sequence Code Division Multiple Access (DS-CDMA) technology, such as the Wideband Code Division Multiple Access (WCDMA) or CDMA2000.
FIG. 1 illustrates a structure of a conventional OFDM signal in the time and frequency domain.
Referring to FIG. 1, one OFDM symbol 100 occupies N sub-carriers 102 in the frequency domain. Each sub-carrier 102 carries an individual modulation symbol 104 for transmission information, and the sub-carriers 102 carrying the modulation symbols 104 are simultaneously transmitted in parallel. Each modulation symbol 104 carried by the sub-carrier 102 is called a “sub-carrier symbol.” As described above, the OFDM technology is multi-carrier transmitting technology, which can transmit data and control channel information through multiple sub-carriers. In FIG. 1, reference numerals 106 and 108 denote the ith and (i+1)th OFDM symbol intervals, respectively. In an OFDM-based communication system, each physical channel includes one or more sub-carrier symbols 104.
An important attribute of an OFDM-based cellular wireless communication system for providing a high speed wireless data service is the ability to support a scalable bandwidth. A system based on a scalable bandwidth can selectively use various bandwidths such as 20/15/10/5/2.5/1.25 megahertz (MHz). In such a system, a service provider can select one of the above-enumerated bandwidths in providing service in each cell, and there may exist various types of User Equipments (UEs) from a UE supporting a maximum of 20 MHz bandwidth to a UE supporting a minimum of 1.25 MHz bandwidth in each cell.
In a scalable bandwidth-based system, it is required of a UE initially accessing the system to be able to successfully search the cells, even though the UE is not aware of the system bandwidth. Through the cell search, the UE acquires a cell IDentifier (ID) and synchronization between a receiver and a transmitter for demodulation of data and control information. It is possible to acquire the system bandwidth either from an SCH during the cell search or through decoding of a Broadcast CHannel (BCH), which is a common control channel for transmission of system information after the cell search.
The BCH is a channel for transmitting system information of a cell that a UE accesses, and is demodulated after the UE terminates the cell search. The UE first performs the cell search through the SCH, and acquires system information of the cell by receiving the BCH after a successful cell search. By reading the BCH, the UE can obtain system information necessary for reception of data channels and other control channels, such as a cell ID of each cell, a system bandwidth and channel setup information.
FIG. 2 illustrates a conventional frequency resource mapping of a BCH and an SCH based on a system bandwidth in a system supporting a scalable bandwidth.
Referring to FIG. 2, the horizontal axis 200 represents the frequency, and the SCH 204 and the BCH 206 are transmitted with a bandwidth of 1.25 MHz regardless of the system bandwidth in the middle of the system bandwidth. Therefore, the UE searches for a Radio Frequency (RF) sub-carrier 202 occupying a central frequency of the system bandwidth regardless of the size of the system bandwidth, and performs cell search based on the SCH 204 for a band of 1.25 MHz centering the RF sub-carrier 202, thereby acquiring initial synchronization. Further, after the cell search, the UE decodes the BCH 206 transmitted in the same 1.25 MHz band, thereby obtaining system information.
An important technical object in a system supporting a scalable bandwidth is to design the channels such that a UE having a bandwidth smaller than the system bandwidth can flawlessly perform BCH reception and cell search by the SCH from neighbor cells, even when the UE uses a service through only a part of the system bandwidth.
Various UEs having different bandwidth-supporting capabilities may exist within a system supporting a scalable bandwidth. For example, FIG. 3 illustrates an example of distribution of UEs 310 to 320 having a reception bandwidth of 10 MHz or 20 MHz in an active mode or idle mode within the system bandwidth.
Referring to FIG. 3, when a minimum reception bandwidth of UEs 310 to 320 capable of accessing the system is 10 MHz, MBMS #1 300 and MBMS #2 302, which are Multimedia Broadcast Multicast Service (MBMS) physical channels, are transmitted in each 10 MHz band within the 20 MHz system band 304. The MBMS channels 300 and 302 are channels for providing a service to multiple users in a single direction through broadcast or multicast, and various broadcast services are provided through the MBMS #1 300 and the MBMS #2 302. Further, the SCH 306 and the BCH 308 are transmitted in the central band centering the RF sub-carrier frequency.
The UE #3 314, which has a minimum bandwidth of 20 MHz and is in the idle mode, can normally receive both the MBMS channels 300 and 302 and the SCH 306 and BCH 308. Further, the UE #4 316, which does not receive the MBMS service and is in the idle mode, occupies a central 10 MHz band of the system band and continuously receives the SCH 306 and the BCH 308 from neighbor cells, so as to perform cell search and system information reception in preparation for entering into the active mode.
In contrast, the UE #1 310 and the UE #2 312, each of which has a reception capability of 10 MHz bandwidth, receive MBMS channels 300 and 302 in the upper and lower half bands including the broadcast channels, respectively. Because the UE #1 310 and the UE #2 312 are also in the idle mode, they also, like the UE #4 316, need to receive not only the MBMS data but also the SCH 306 and the BCH 308 from neighbor cells, in preparation for entering into the active mode. However, each of the UE #1 310 and the UE #2 312 receives only a part of the SCH 306 and only a part of the BCH 308. With the SCH 306, it is possible to perform the cell search by using only a sequence of a part of the band of the SCH 306. However, with the BCH 308, it is impossible to normally decode the system information without receiving all sub-carrier symbols of the band of the BCH 308. Likewise, the UE #5 318 and the UE #6 320, which are in the active mode in the upper and lower half bands, have the same problem.
In order to enable a normal decoding of the BCH 308, each of the UE #1 310 and the UE #2 312 needs to change its own reception RF frequency to the band (BCH transmission band) in which the BCH 308 is transmitted, receive the BCH 308, and then recover its original half band in which the MBMS channels 300 or 302 are transmitted. However, during such a process, it is difficult to achieve seamless reception of the MBMS data and cell search. Therefore, it is necessary to design the SCH and the BCH which enable UEs to smoothly move between cells without changing the reception RF frequency.
Particularly, a cellular communication system can operate in a Frequency Division Duplex (FDD) mode and a Time Division Duplex (TDD) mode. Therefore, there is a need for a downlink synchronization channel structure, which can be applied to both the FDD and the TDD and enables a UE to seamlessly perform initial cell search and neighbor cell search.